Compact, highly sensitive, rapid continuous measurement of carbon dioxide released from fruits and vegetables based on QEPAS and TDLAS dual spectrum multiplexing technology

This paper presents the development of a compact, highly sensitive and rapid continuous measurement CO₂ sensor based on the multiplexing of quartz-enhanced photoacoustic spectroscopy (QEPAS) and tunable diode laser absorption spectroscopy (TDLAS). The sensor utilizes a distributed feedback (DFB) laser diode with wavelength of 2004 nm as the excitation source, while the photoacoustic signal is generated using a commercially available, low-cost 30.720 kHz quartz tuning fork (QTF). The fiber-coupled grin collimator, QTF, acoustic micro-resonator, CO₂ reference cell, photodiode, gas chamber, thermistor, and Peltier were all integrated to create an ultra-compact acoustic detection module (ADM) with dimensions of 44.50 × 26.00 × 17.50 mm. In dual spectrum multiplexing technique, the detection limit of CO₂ was 8.7 ppm at 1 s, corresponding to a normalized noise equivalent absorption coefficient of 6.20 × 10^–9 W·cm^-1·Hz^-1/2. The long-term performance and stability of the QEPAS CO₂ sensor were evaluated through Allan deviation. Eventually, the minimum detection limit (MDL) increased to 0.7 ppm at 228 s. The QEPAS CO₂ sensor was developed for fast and highly sensitive measurements of CO₂ respiration rate in different fruit species. Among the six kinds of fruit and vegetable, edamame exhibited the highest CO₂ respiration rate, with an average of 55.16 ± 0.67 mL kg⁻¹ h⁻¹, while red grapes showed the lowest rate, averaging 6.12 ± 0.09 mL kg⁻¹ h⁻¹.